kes Move lock structures into new file lock.h

kes  Use jcr lock for locking reservations messages rather than
     the reservations lock.
kes  Rename a number of locking functions to make their sense a
     bit more logical (more to be done), also rename a few other
     methods in the DEVICE structure.
kes  Add a good bit of documentation of the SD locking mechanisms
     to src/stored/lock.c
kes  Convert a number of bnet_xx calls to use the new bsock class.